The invention relates to a cathode-ray tube having an envelope comprising a metal rear housing and a glass face-plate which is sealed in a vacuum-tight manner to a flange on the rear housing by means of a seal formed between the face-plate and the flange. The seal may be a pressure-bonded seal consisting of a pressure-deformable material, or it may be formed by an adhesive which bonds the face-plate to the flange.
The rear housing, which contains the electron gun and other components of the cathode-ray tube, is usually referred to as the cone portion of the envelope, although it may not be strictly, or even remotely, conical in the geometric sense. For example, a new type of rear housing which is very relevant to the present invention and which may be referred to as a "flat-can" rear housing has the form of a shallow, rectangular, flat-bottomed metal can with an open top surrounded by a flange. An envelope having this type of rear housing is used in flat cathode-ray tubes. To form the envelope a rectangular glass face-plate is sealed to the flange of the can.
In the manufacture of a cathode-ray tube having an envelope of the construction described in the opening paragraph above, when the envelope is evacuated the face-plate deflects inwards slightly and there is a consequent tendency for the face-plate to peel away from the seal between the face-plate and the flange of the rear housing and/or for the seal to peel away from the flange. To counteract this tendency the Applicants have already proposed in their prior United Kingdom Patent Specification No. 1,598,888, which is concerned with a cathode-ray tube of the kind in which the edges of the face-plate and the edges of the flange of the rear housing have a slight convex curvature, to clamp the face-plate to the flange by means of channel members which are slightly curved to conform to the curvature of the edges of the face-plate and the flange and which are forced onto these edges by a metal rim-band which extends around the periphery of the face-plate and flange. The rim-band can be tightened by means of a draw-bolt or it may be an endless band shrunk onto the channel members. Either way the resulting longitudinal tensile stress in the band, due to the curvature of the channel members, produces a force on these members which drives them tightly onto the mating edges of the face-plate and the flange. One side wall of each of the substantially U-section channel members lies flat against the outer surface of the face-plate; the other side wall diverges slightly with respect to the first side wall so as to exert a wedging action on the flange which urges the flange towards the face-plate and constrains it to follow the deflection of the face-plate when the envelope is evacuated.
Since the rim-band operates by exerting on the channel members forces which are directed transversely of these members towards the inside of the envelope, the combination of rim-band and channel members can be used only on envelopes in which the edges of the face-plate and the flange of the rear housing have a convex curvature. In view of the trend towards flat cathode-ray tubes here is a growing need for a clamping means which can be used on envelopes in which the face-plate and the flange are of rectangular shape with straight edges. It is an object of the invention to satisfy this need.